The present invention relates to, inter alia, radiation-sensing detectors comprising one or more individual detector elements (xe2x80x9cpixelsxe2x80x9d) each including a thermally displaceable member. Incident radiation such as infrared radiation is locally absorbed and converted to heat by the thermally displaceable elements, causing the thermally displaceable elements to individually exhibit a corresponding thermal displacement. The displacements impart corresponding changes to a signal light or other detectable entity.
Conventional infrared-sensor panels include an array of a large number of individual sensor elements (xe2x80x9cpixelsxe2x80x9d). Each pixel comprises a membrane portion that includes a planar surface made from one or more membrane layers. The membrane portion is suspended in space relative to a respective substrate in the manner of, e.g., a cantilever, and typically is made using micro-machining technology such as technology used in the manufacture of semiconductor integrated circuits.
For example, one type of conventional pixel in such an array includes a displaceable membrane member that is supported relative to a substrate by a leg portion. The displaceable membrane member includes a radiation-absorbing region. As the radiation-absorbing region absorbs incident electromagnetic radiation (e.g., infrared (IR) radiation), the displaceable membrane member exhibits a corresponding displacement relative to the substrate. Such elements have been incorporated into capacitor-based as well as light-based thermal-type IR detectors. See, e.g., U.S. Pat. Nos. 3,896,309 and 5,844,238; and U.S. patent application Ser. No. 08/994,949, now U.S. Pat. No. 6,080,988. In other conventional applications, the membrane member has been incorporated into the leg portion of a thermal-type displaceable element. A capacitor-based radiation detector reads out displacements to individual constituent pixels (caused by incident radiation) as respective changes of capacitance. A light-based radiation detector reads out displacement to individual constituent pixels (caused by incident radiation) as respective changes in a read-out (signal) light.
According to conventional practice, the membrane member typically includes a planar-surface portion formed from at least one layer having a desired planar configuration. In such a configuration, it is desirable that the membrane portion be as thin as possible to reduce the mass of the displaceable membrane member and thus to improve responsivity. However, conventional approaches exploited to achieve this end are not satisfactory.
In a radiation detector such as an IR detector, the rate at which incident radiation is absorbed by each pixel desirably is high to enhance detection sensitivity. It also is desirable that the thermal capacity of the radiation-absorbing region of each pixel be as small as practicable to enhance detection response. It also is desirable that the heat generated in each radiation-absorbing region be efficiently and effectively conducted to the respective displaceable member to enhance detection sensitivity.
In a conventional radiation detector, even if a gold-black membrane exhibiting a relatively high rate of radiation absorption rate is used as the radiation-absorbing region, it has not been possible to date to enhance both detection sensitivity and detection response of the pixels. More specifically, the rate of radiation absorption exhibited by gold black is, for example, about 960 cmxe2x88x921 with incident IR radiation having a wavelength in the range of 8 to 12 xcexcm. Zaeschmar and Nedoluha, xe2x80x9cTheory of the Optical Properties of Gold Blacks,xe2x80x9d J. Am. Opt. Soc. 62(3):348-352, March 1972. The efficiency with which gold black (formed as a 1-xcexcm thick membrane) absorbs infrared radiation is only about 9%. Increasing the efficiency can be achieved by increasing the thickness of the gold-black membrane. However, this remedy leads to other problems. Conventionally, it is impossible both to reduce the membrane thickness while simulaneously enhancing the rate of absorption of IR radiation simply by forming the membrane in a manner such that the incident radiation merely enters the membrane. Hence, it is currently impossible to enhance both detection sensitivity and detection response of these detectors.
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide, for a radiation detector, a displaceable structure of which the thickness (and thus the mass) can be reduced while maintaining a desired mechanical strength of the displaceable structure. Another object is to provide a thermal-type displaceable element, and a radiation detector comprising one or more such elements, exhibiting enhanced image-sensing performance while maintaining a desired mechanical strength and reducing the thickness of the displaceable element.
According to one aspect of the invention, a radiation detector is provided that includes a displaceable member. The displaceable member comprises a planar portion comprising at least one membrane layer. The planar portion is supported so as to be suspended over a substrate of the detector. The planar portion includes a xe2x80x9cdropping portionxe2x80x9d or xe2x80x9crising portionxe2x80x9d extending along at least a portion of the periphery of the planar portion. The planar portion can be fabricated using a semiconductor-fabrication process. The dropping or rising portion desirably is formed of at least one layer of the planar portion.
By placing the dropping or rising portion around at least a portion of the periphery of the planar portion, the planar portion is structurally reinforced by the dropping or rising portion. This structural reinforcement allows the thickness (and thus the mass) of the planar portion to be reduced without compromising its mechanical strength. Also, the dropping or rising portion helps maintain uniformity of the planar portion, even if the planar portion is formed of multiple membrane layers.
If the planar portion is formed of multiple membrane layers, it can be peripherally edged in a manner in which at least one of the layers covers the peripheral edge of at least one of the other layers. Such a structure can be fabricated using a semiconductor-fabrication process. Because the planar portion is structurally reinforced by the covered edge of at least one layer, the thickness (and thus the mass) of the planar portion can be reduced without compromising the mechanical strength of the planar portion.
Furthermore, even if the planar portion would otherwise tend to exhibit a displacement due to any difference in the coefficients of thermal expansion of the layers making up the planar portion, such displacement is arrested by the strength imparted by the covered peripheral edges.
A thermal-type displaceable element of a radiation detector according to the invention can comprise a leg serving to connect the displaceable element to the substrate and to suspend the displaceable element over a corresponding region of the substrate. The leg can comprise at least one membrane layer and can be fabricated as an extension of the planar portion. The leg desirably has a thermal-insulation property. The greater the thermal insulation provided by the leg, the greater the displacement that can be imparted to the displaceable element. The leg also can include a dropping portion extending around a planar portion of the leg, so as to provide the leg with enhanced structural rigidity while allowing the membrane thickness of the leg to be reduced. By reducing the thickness of the leg, its thermal-insulation property can be enhanced.
The radiation detector can comprise a thermal-type displaceable element comprising a displaceable portion that is displaced according to heat generated by incident light (e.g., IR light). The displaceable portion is displaced (e.g., tilted) by an amount corresponding to the amount of generated heat. As an alternative to IR radiation, the detector can be configured to undergo heating in response to other wavelengths of light such as X-rays, ultraviolet rays, etc. By maintaining good thermal insulation between the substrate and the displaceable portion, the actual displacement exhibited by the displaceable portion is an accurate function of the amount of incident radiation actually received, thereby providing an improved signal-to-noise (S/N) level to the detector.
In a representative embodiment, the radiation detector includes a substrate, a displaceable member supported relative to the substrate and exhibiting a displacement relative to the substrate in response to heat. The displaceable member includes a displacement readout member fixed thereto. The displacement readout member is used for measuring the displacement of the displaceable member. The displacement readout member can be arranged at a predetermined distance from the displaceable member and can be formed from the same membrane layer(s) as the displaceable member. By forming the displacement readout member from the same membrane(s) as the displaceable member, the thickness of the displacement readout member can be reduced while maintaining the mechanical strength of the displaceable readout member.
The displaceable member typically comprises at least two mutually overlapping layers of different substances having different coefficients of thermal expansion. The thinner the layers, the greater the displacement that can be achieved with a given change in temperature. Thus, the sensitivity of the detector is correspondingly increased.
The displacement readout member can be a reflector configured to reflect incident readout light. Alternatively, the displacement readout member can be an electrode of a capacitor structure. A configuration including a reflector is termed a xe2x80x9clight-readout radiation detector.xe2x80x9d A configuration including an electrode is termed a xe2x80x9ccapacitor-type radiation detector.xe2x80x9d These configurations, however, are not to be construed as limiting in any way.
A radiation detector according to the invention normally includes multiple detection elements (xe2x80x9cpixelsxe2x80x9d). The pixels can be arranged in a one-dimensional or two-dimensional array. A two-dimensional array is especially useful for detecting an image in the incident radiation.
According to one embodiment, a radiation detector according to the invention can be formed as follows. Each pixel comprises a respective xe2x80x9cdisplaceable structurexe2x80x9d comprising multiple independently displaceable members configured as a multiple-stage displaceable structure. Each of the independently displaceable members is linearly extended and includes two or more mutually overlapping layers of different substances having different thermal expansion coefficients. The independently displaceable members are arranged parallel to each other. The ends of the independently displaceable members are connected either to the substrate or to xe2x80x9cconnecting membersxe2x80x9d so as to form a single integral (but nevertheless multiple-stage) mechanical linkage from the substrate to a displaceable readout member supported by the displaceable structure. With such a configuration, large displacements can be obtained with minimal space per pixel on the substrate.
Alternatively, the displaceable structure of each pixel can comprise only a single displaceable element. However, by configuring the displaceable structure to have multiple independently displaceable members connected together using connecting members as summarized above, a displacement substantially the same as otherwise obtainable with a displaceable structure including only a single independently displaceable member having a length equal to the total length of the independently displaceable members can be obtained. Hence, the configuration including multiple independently displaceable members provides a greater degree of freedom.
The connecting members can be configured to have at least one membrane layer and can be configured with strength-enhancing features as summarized above. Such configurations allow the membrane thicknesses of the connecting members to be reduced without compromising mechanical strength of the connecting members. This allows the mass of the connecting members to be reduced, thereby allowing the mass of independently displaceable members connected thereto to be reduced. These mass reductions provide enhanced detector sensitivity.
A thermal-type displacement element according to another embodiment includes a displaceable structure mounted to the substrate and that is displaceable relative to the substrate in response to heat. The displaceable structure includes multiple independently displaceable members. Each independently displaceable member extends in a straight line and includes two or more mutually overlapping layers of different substances having different thermal expansion coefficients. The independently displaceable members are arranged in parallel. Certain independently displaceable members are affixed to the substrate via a respective leg. Other independently displaceable members are connected to the members connected to the substrate or to each other to form a single intregral mechanical connection. Each leg can be composed of a membrane layer. With such an embodiment, even if plural pixels are arranged on the substrate, large displacements can be obtained with minimal substrate space being occupied by each pixel.
In the foregoing configuration, the legs can be formed from the membrane structure of an independently displaceable element. This allows the layer thickness of the leg to be reduced without compromising mechanical strength of the leg. This, in turn, provides for improved thermal insulation by the leg between the substrate and the displaceable structure.
Furthermore, the coefficients of thermal expansion of the layers comprising the independently displaceable members desirably are arranged in opposite order with each xe2x80x9cstagexe2x80x9d of the linkage. With such a configuration, the total displacement is about the same magnitude as it otherwise would be if the displaceable structure only comprised one independently displaceable member having a length equal to the combined lengths of such members in the multiple-stage configuration.
In the multiple-stage configuration, the final stage can terminate with a displacement readout member exhibiting a change to a readout medium (e.g., readout light) commensurate with the amount of displacement exhibited by the displaceable structure. In addition, the independently displaceable members in the displaceable structure can be configured to absorb incident radiation and generate heat that effects the displacement of the structure.
A thermal-type displaceable element as summarized above, or any of various other configurations of such elements according to the invention, need not necessarily be used in a radiation detector. They alternatively can be used as a simple sensor for detecting, for example, a temperature distribution.
Furthermore, although a displaceable structure as summarized above is especially suitable for use in a thermal-type displaceable element or in a radiation detector, the structure also can be used for any of various other applications such as micro-machines.
According to yet another aspect of the invention, radiation detectors are provided that include a substrate and a displaceable structure supported relative to the substrate. The displaceable structure includes a radiation-absorption member configured to absorb an incident radiation, to undergo heating from such absorption, and to exhibit a corresponding displacement from such heating. Desirably, the radiation-absorption member reflects a portion of the incident radiation. The detector also comprises a radiation-reflection member situated at a distance nxcex0/4 from the radiation-absorption member, wherein n is an odd integer and xcex0 is the median wavelength of a desired range of wavelengths of the incident radiation. The radiation-reflection member totally reflects the incident radiation. With this configuration, the incident radiation need not be infrared (IR) radiation, but alternatively can be any of various other types of electromagnetic radiation such as X-rays, ultraviolet rays, and the like.
The radiation detector can be configured as a light-readout-type radiation detector, for reading out a displacement caused in a displaceable structure as a change in readout light irradiated separately onto the detector. Alternatively, the radiation detector can be configured to read out displacements as corresponding changes in capacitance across respective electrodes. The latter configuration is termed a xe2x80x9ccapacitance-typexe2x80x9d detector.
In the case of the light-readout-type radiation detector, a readout-light reflector member can be provided with the displaceable structure for measuring the displacement of the displaceable structure. For example, a semitransparent mirror can be provided on the displaceable structure to both reflect and transmit readout light. A readout-light reflector also can be provided on the substrate so as to face the semi-transparent mirror. The displaceable structure also includes an absorbing region to absorb incident radiation. A part of the incident radiation is absorbed by the absorbing region. Remaining incident radiation is reflected by an incident-radiation reflector back to the absorbing region. Thus, an interference is established between the absorbing region and the incident-radiation reflector as a result of the distance between these structures being defined as noted above. The amount of absorbed incident radiation by the absorbing region is maximized using such a structure. Even if the thickness of the absorbing region is reduced and thermal capacity decreased, the absorption efficiency of incident radiation is still increased, thereby increasing detection sensitivity and detection responsiveness.
In the case of a capacitor-type radiation detector, the movable electrode can be provided on the displaceable structure, and a fixed electrode can be provided on or in the substrate so as to face the movable electrode.
In any event, the efficiency of absorption of incident radiation is further increased if the reflectivity of the incident-radiation absorbing region is about 33% (i.e., about ⅓), which is desirable.
Radiation is absorbed and thus heat is generated from the incident-radiation reflector based on the interference phenomenon summarized above. However, the volume of radiation absorbed by the radiation absorber is greater than the amount of radiation absorbed by the radiation reflector. Thus, since the displaceable structure includes the incident-radiation absorber (in contrast to placing the radiation absorber on the substrate and the radiation reflector on the displaceable structure), a large displacement of the displaceable structure is achieved with the same amount of incident radiation, thereby increasing detection sensitivity.
Whenever the radiation absorber is affixed to the substrate via a thermal-insulation member not provided to the displaceable structure, and heat generated in the radiation absorber is conducted to the displaceable structure, heat generated in the radiation absorber passes to the substrate through the thermal-insulator because it is impossible to insulate heat perfectly. Thus, detection sensitivity could be reduced since heat is not being effectively conducted to the displaceable structure. But, by placing the radiation absorber on the displaceable structure, heat generated in the radiation absorber is effectively conducted to the displaceable structure, thereby enhancing detection sensitivity.
The radiation reflector may be provided on the displaceable structure in order for a relative-positional relationship between the radiation reflector and the radiation absorber to be maintained substantially constant, notwithstanding any displacement of the displaceable structure.
It is not required that the radiation reflector be provided on the displaceable structure. It is possible to obtain a stable spectral response characteristic even if the radiation reflector is provided on the displaceable structure (in order for the relative relationship between the radiation reflector and the radiation absorber to be kept in a constant condition), notwithstanding displacement of the displaceable structure. Namely, since the relative relationship between the radiation reflector and the radiation absorber is kept in a constant condition notwithstanding displacement of the displaceable structure, absorption of radiation by the radiation absorber occurs in accordance with the above-described interference principle. Hence, it is possible to keep the wavelength range of radiation absorbed in the radiation absorber in a constant condition.
A readout-light reflector reflecting received readout light may be included on the displaceable structure. The readout-light reflector can serve both as an incident-radiation reflector and a readout-light reflector or can be formed as a readout-light reflector only.
For a readout-light-type radiation detector, it is possible to provide a simple structure inexpensively by forming the radiation reflector to serve both as an incident-radiation reflector and a readout-light reflector.
A fixed electrode may be provided in the substrate and a movable electrode may be provided in the displaceable structure so as to face the fixed electrode. Thus, the radiation reflector may serve both as an incident-radiation reflector and as a movable electrode, or can be formed in the movable electrode.
For a capacitor-type radiation detector, it is possible to provide a simple structure inexpensively by forming the radiation reflector to serve both as the incident-radiation reflector and as the movable electrode.
A fixed electrode can be provided in the substrate and a movable electrode can be provided in the displaceable structure so as to face the fixed electrode. Thus, the radiation reflector can serve both as an incident-radiation reflector and a fixed electrode.
The fixed electrode can be provided in the substrate and a movable electrode can be provided in the displaceable structure so as to face the fixed electrode. Thus, the radiation reflector can serve as both an incident-radiation reflector and as a movable electrode.
When at least one of the radiation absorber and the radiation reflector is provided in the displaceable structure by a membrane member having a planar portion composed of one or more membrane layers and supported such that the planar portion is suspended in the air, it is preferable to form a rising portion extending from the planar portion, or a dropping portion extending from the planar portion around at least a portion of the periphery of the planar portion. In another configuration, when at least one radiation absorber and a radiation reflector is provided in the displaceable structure formed by a membrane member having a planar portion composed of plural layered membranes and supported in such a way that the planar portion is suspended in the air, it is preferable to form one or more layered membranes of the plural layered membranes so as to cover a different one or more layered membranes of the plural layered membranes around at least part of the periphery of the planar portion. In such cases, the planar surface of one or both of the radiation reflector and the radiation absorber provided in the displaceable structure is reinforced by the rising portion, the dropping portion, or the portion in which one or more layered membranes cover an edge portion of different one or more layered membranes. In such a configuration, the membrane thickness of the planar portion can be reduced without compromising strength. Thus, it is possible to decrease the thermal capacity while preventing deformation caused by excessive structural weakness. As a result, the detection response is enhanced while providing a more stable spectral response characteristic. This is achieved in part by maintaining constancy of the distance between the radiation absorber and the radiation reflector that would otherwise arise due to such deformation.
The foregoing and other features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.